This invention relates to steam generators and more particularly to methods for removing sludge deposits from the tubesheets of steam generators.
A typical nuclear steam generator comprises a vertically-oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tubesheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tubesheet forming a primary fluid inlet plenum at the one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the primary fluid inlet plenum, and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet plenum. The steam generator also comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber adjacent the shell and a feedwater ring disposed above the U-like curvature end of the tube bundle. The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid is conducted through the primary fluid inlet plenum, through the U-tube bundle, out the primary fluid outlet plenum, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator through the feedwater ring. The feedwater is conducted down the annular chamber adjacent the shell until the tubesheet near the bottom of the annular chamber causes the feedwater to reverse direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the feedwater is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the feedwater surrounding the tubes causing a portion of the feedwater to be converted steam. The steam then rises and is circulated through typical electrical generating equipment thereby generating electricity in a manner well known in the art.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the U-tube walls which may be constructed by Inconel, the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no breaks will occur in the U-tubes. However, experience has shown that under certain circumstances, the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater, a highly undesirable result.
There is now thought to be at least two causes of tube leaks in steam generators. One cause of these leaks is considered to be related to the chemical environment of the feedwater side of the tubes. Analysis of the tube samples taken from operating steam generators which have experienced leaks has shown that the leaks were caused by cracks in the tubes resulting from intergranular corrosion. High caustic levels found in the vicinity of the cracks in the tube specimens taken from operating steam generators and the similarity of these cracks to failures produced by caustic under controlled laboratory conditions have identified high caustic levels as the cause of the intergranular corrosion and thus the cause of the tube cracking.
The other cause of tube leaks is thought to be tube thinning. Eddy current tests of the tubes have indicated that the thinning occurs on tubes near the tubesheet at levels corresponding to the levels of sludge that has accumulated on the tubesheet. The sludge is mainly from oxides and copper compounds along with traces of other metals that have settled out of the feedwater onto the tubesheet. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and the tube wall thinning location strongly suggests that the sludge deposits provide a site for concentration of the phosphate solution or other corrosive agents at the tube wall that results in tube thinning.
One method for removing sludge from a steam generator is described in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System," issued Mar. 21, 1978 in the name of Hickman et al. and assigned to the Westinghouse Electric Corporation.
In many nuclear steam generators in service today, there are 6 inch diameter hand holes in the shell of the steam generator near the tubesheet that provide access to the tubesheet for removal of the sludge deposits on the tubesheet. However, many of the steam generators in service today do not have 6 inch diameter hand holes near the tubesheet; rather, they may have 2 inch diameter inspection ports near the tubesheet which greatly limit the access that may be had to the tubesheet. This limited access greatly limits the types of apparatus and methods that may be used to remove the sludge from the tubesheets in the steam generators.
Therefore, what is needed is a method for removing sludge deposits that is capable of being used on steam generators having the 2 inch diameter inspection ports for access to the tubesheets.